This invention relates to a superconducting composite, and more particularly to superconducting composites comprising a high strength, ultrahigh modulus, high thermal conductivity carbon substrate and a ceramic-type superconductor layer. In one embodiment, the invention relates to superconducting composites comprised of a high strength, ultrahigh modulus, high thermal conductivity, low electrical resistivity carbon fiber disposed within a sleeve layer formed of ceramic-type superconductor such as a rare earth, Ba, Cu, oxide-type superconductor (1-2-3 superconductor), which composite is capable of achieving significant current densities at high magnetic field strengths under superconducting conditions. The term carbon fiber as used herein includes both a carbon monofilament as well as a bundle of monofilaments (a yarn).
Superconducting ceramic-type materials composed of a combination of rare earth (e.g. yttrium) oxide, barium oxide and copper oxide which have significantly higher superconduction transition temperatures than earlier materials such as Nb/Ti alloys, niobium carbonitride and the like are widely known and well described in the art. Superconduction transition temperatures above 77.degree. K. (the boiling point of liquid nitrogen) are commonly found for these materials, and even higher transition temperatures are considered possible based upon current theories explaining superconducting behavior. The economic advantage that these new superconductors could have over previously existing lower superconducting-transition-temperature superconductors is large enough that many new uses for superconductors now can be devised and present uses enormously improved. However, because these new mixed-oxide superconductors are brittle, ceramic-like materials, they do not lend themselves easily to fabrication in the form of high strength, wire-type geometries, a requirement for many important uses to which superconductors have been put in the past. These uses largely revolve about strong field magnets used in high energy physics, traffic engineering, etc.
One way of fabricating a brittle superconducting material in wire-like form is set forth in an article by K. Brennfleck et al. entitled "Chemical Vapor Deposition of Superconducting Niobium Carbonitride Films on Carbon Fibers", published in Proceedings of the 7th Conference on Chemical Vapor Deposition, Electrochemical Society (1979) at p. 300. This article describes depositing a niobium carbonitride layer directly onto a multifilament carbon yarn by chemical vapor deposition (CVD) to form a superconducting composite. However, the Brennfleck et al. composites employ a low thermal conductivity, more reactive carbon fiber and the structure shown in the photomicrographs accompanying the article exhibits a poor physical structure. Addition al aspects of niobium carbonitride-carbon fiber based superconducting composites are taught in U.S. Pat. Nos. 4,299,861; 4,581,289; and 4,657,776. Ultrahigh modulus, high thermal conductivity carbon fibers of low resistivity that will perform most, if not all, the stabilization required for carbon fiber superconducting composite operation are disclosed in U.S. Pat. No. 5,266,294. Thus, the need for the outermost copper coating used in the previous literature for stabilization is either reduced or eliminated resulting in simpler and more economical devices.
The usefulness of an intermediate carbide or oxide layer between a carbon fiber and a niobium carbonitride superconductor layer to improve adhesion of the superconductor is taught in U.S. Pat. No. 4,585,696. Such a layer depends upon its intermediate (to the fiber and superconductor) coefficient of expansion to achieve its adhesive effect.
The new mixed-oxide ceramic-type suprconductors are different in physical properties than the Brennfleck et al. niobium carbonitride material and these differences lead to different considerations for fabricating the superconductor into wire-like form. For example, the niobium compound has a cubic crystal structure and its critical current and critical fields are isotropic, i.e., the same along each of its three crystallographic axes. The new 1-2-3 superconductors on the other hand show a much smaller critical current and critical field along the c crystallographic axis than along the a and b crystallographic axes. Thus, it may be important to align the a b planes of the 1-2-3 superconductor microcrystals as completely as possible parallel to the fiber axis for maximum effectiveness when made in a superconducting device.
More recently, in U.S. Pat. No. 4,975,413 there were disclosed superconductive composites comprising a ceramic-type superconductor such as the rare earth, Ba, Cu oxide-type superconductors formed as an adhering layer on a low electrical resistivity, high thermal conductivity, high strength, ultrahigh modulus carbon fiber. The composites are disclosed to be formable into strong, flexible conductors capable of exhibiting substantial critical currents and critical magnetic fields under superconducting conditions. Although these composites are thus quite attractive for superconductive uses, there are substantial differences in coefficient of thermal expansion between the coated superconductors and the carbon substrate. The differential expansion and contraction that results in the various manufacturing steps, and particularly when coating the carbon fiber with the ceramic at an elevated temperature and then cooling the composite, places significant stress on the structure which may lead to cracking and other failure modes. As is further disclosed in the patent, a compressive layer may be included between the coating and the substrate to accommodate these dimensional changes and thereby minimize such failures during the manufacturing operations or avoid them altogether. However, when the composite is repeatedly cycled in use to the very low temperatures necessary for superconduction, the dimensional changes may be quite large, placing stresses on the ceramic coating that are very severe and raider exacerbate the cracking problem. The art thus continues to seek further improvement in the design of composite structures that are intended to withstand thermal cycling to the extremely low temperatures contemplated as the operating environment for superconductive structures.